This invention relates to an oxygen concentration-sensing device and the method of producing the same, and more particularly to an oxygen concentration-sensing device for sensing the oxygen concentration in the exhaust of an internal combustion engine.
To cope with the exhaust of an internal combustion engine, oxygen concentration-sensing devices are widely known. This device is applied to determine the air-fuel ratio of a gas mixture taken into the combustion chamber of an internal combustion engine from the level of oxygen concentration in the exhaust. When, therefore, the air-fuel ratio of a gas mixture is so controlled by said device as to conform to a theoretical air-fuel ratio, it is possible to reduce the amount of harmful gas components such as CO, HC, NOx contained in the exhaust. Further, such a device can control the air-fuel ratio of the gas mixture to be greater than the theoretical air-fuel ratio, thereby decreasing fuel consumption.
Description will now be given of the fundamental construction of the above-mentioned oxygen concentration-sensing device. This device which is formed of an oxygen-ion-permeable metal oxide comprises a solid electrolyte body having a first surface exposed to a gas to be sensed and a second surface exposed to a reference gas, a first electrode fixed to the first surface of the solid electrolyte body, a second electrode fixed to the second surface of the solid electrolyte body, and a gas diffusion-resistant layer coated on the first electrode.
Description will now be given of the operation of the oxygen concentration-sensing device constructed as described above. When a voltage is impressed between the first and second electrodes, oxygen ions of an amount corresponding to the oxygen concentration in the exhaust diffuse through the solid electrolyte body, causing a current corresponding to the oxygen concentration in the exhaust to flow across both electrodes. When a higher voltage than prescribed is impressed across both electrodes, the current flowing through both electrodes remains substantially constant regardless of the magnitude of the voltage. The substantially constant current is referred to as a saturated current. Therefore, measurement of this saturated current indicates the oxygen concentration in the exhaust.
It is known that the value Il of the saturated current can be determined from the following formula: ##EQU1## where: F=Faraday constant
R=gas constant PA1 DO.sub.2 =diffusion constant of oxygen molecule PA1 T=absolute temperature of a solid electrolyte body PA1 E=rate of gas (oxygen molecule) diffusion through a gas diffusion-resistant layer PA1 l=effective gas diffusion distance in the gas diffusion-resisting layer PA1 S=electrode area PA1 PO.sub.2 =partial pressure of oxygen molecule
As seen from the above formula, the value Il of the saturated current varies due to different factors. If, therefore, it is attempted to exactly determine the value Il of the saturated current corresponding to the oxygen concentration in the exhaust, it is necessary to accurately define the different factors. Attention must be paid particularly to the extent to which the saturated current Il is affected by the various factors except for the above-mentioned constants, namely, the absolute temperature T and the gas diffusion rate E in the gas diffusion-resistant layer.
In this connection, reference is made to the U.S. Pat. No. 4,356,065 which sets forth the effect of the diffusion rate E in the gas diffusion-resistant layer on the value Il of the saturated current and the process of eliminating said effect. However, U.S. Pat. No. 4,356,065 pays attention only to the diffusion rate E in the gas diffusion-resistant layer, and pays no heed to the absolute temperature T indicated in the aforementioned formula.
The present inventors investigated the effects of absolute temperature T on the value Il of the saturated current. As a result, it has been disclosed that the relationship shown in FIG. 1 exists between the value Il of the saturated current and the absolute temperature T. It is obvious that the value Il of the saturated current varies with the absolute temperature T. For the precise determination of the value Il of the saturated current, therefore, it is necessary to maintain the temperature of the solid electrolyte body at a substantially constant level. However, considerable difficulties are encountered in setting the temperature of the solid electrolyte body at a constant level by means of, for example, a heater element. If it is tried to preserve the constant temperature of the solid electrolyte body by means of the heater element, then the resultant device will have a very complicated arrangement.